This invention relates to surgery in which an implant is placed in the medullary canal of a bone, and more particularly to plugs for insertion in the medullary canal in such surgery.
An implant has to be placed in the medullary canal of a femur when a patient is to be given an artificial hip joint, in order to locate the hip prosthesis. The implant is located in the medullary canal by use of a bone cement, e.g. polymethylmethacrylate cement. Such cement does not have any appreciable adhesive qualities, and relies for its security on obtaining a mechanical interlock with irregularities in the bone. Such mechanical interlocking is greatly enhanced if the cement is introduced into the canal under pressure. However, there is a tendency for the cement to flow down the medullary canal so it is not possible to use much pressure. To overcome this, it is known to insert a plug which is a push fit inside the medullary canal and which prevents the cement flowing down the canal and permits greater pressurisation of the cement. It is known for such plugs to be formed from bone, or from bone cement. An improved plug made from a plastics material is the Seidel plug. Such a plastics plug will of course remain inside the medullary canal when the surgery is completed. A problem with all such known plugs is that because of such factors as the difference in Young's modulus between the bone and the bone cement, there is an abrupt change in the flexural rigidity of the bone at the plug. This has not uncommonly led to fractures at the level of the plug, for example if the patient should suffer a fall. A fracture at this level is very difficult to set.